Magnetic resonance imaging (MRI) is a technique which utilizes the nuclear magnetic resonance phenomenon (NMR) for discovering the local distributions of the nuclear density and nucleus-related NMR characteristics of an object and the physical and chemical characteristics affecting the same. Said NMR characteristics include e.g.: longitudinal relaxation (characterized by longitudinal relaxation time T1), transverse relaxation (characterized by transverse relaxation time T2), relaxation in a rotating frame of reference (characterized by relaxation time T1rho), chemical shift, nuclear coupling factors and, physical phenomena affecting the NMR characteristics, such as: flow, diffusion, paramagnetic substances, ferromagnetic particles, viscosity and temperature.
Methods and applications of magnetic resonance imaging have been described in a number of references: Poole CP and Farach HA: Theory of magnetic resonance, John Wiley, New York, 1987, Stark DD and Bradley WG: Magnetic resonance imaging, C. V. Mosby Comp., St. Louis 1988, Gadian DG: Nuclear magnetic resonance and its applications to living systems, Oxford Univ. Press, London 1982, Shaw D: Fourier transform NMR spectroscopy, Elsevier, Amsterdam, 1984, Battocletti JH: NMR proton imaging, CRC Crit. Rev. Biomed. Eng. vol. 11, pp. 313-356, 1984, Mansfield P and Morris PG: NMR imaging in biomedicine, Adv. in magnetic resonance, Academic Press, New York 1982, Abragam A: The principles of nuclear magnetism, Clarendon Press, Oxford 1961, Lasker SE and Milvy P (eds.): Electron spin resonance and nuclear magnetic resonance in biology and medicine and magnetic resonance in biological systems, Annals of New York Academy of Sciences vol. 222, New York Academy of Sciences, New York 1973, Sepponen RE: Discrimination and characterization of biological tissues with magnetic resonance imaging: A study on methods for T1, T2, T1rho and chemical shift imaging, Acta Polytechnica scandinavica EL-56, Helsinki 1986, Fukushima E and Roeder SB: Experimental pulse NMR, Addison Wesley, London 1981, Anderson W et al: U.S. Pat. No. 3,475,680, Ernst RR: U.S. Pat. No. 3,501,691, Tomlinson BL et al: U.S. Pat. No. 4,034,191, Ernst RR: U.S. Pat. No. 3,873,909, Ernst RR: U.S. Pat. No. 4,070,611, Bertrand RD et al: U.S. Pat. No. 4,345,207, Young IR: U.S. Pat. No. 4,563,647, Hofer DC et al: U.S. Pat. No. 4,110,681, Savelainen MK: Magnetic resonance imaging at 0.02 T: Design and evaluation of radio frequency coils with wave winding, Acta Polytechnica Scandinavica Ph 158, Helsinki 1988, Sepponen RE: U.S. Pat. No. 4,743,850, Sepponen RE: U.S. Pat. No. 4,654,595, Savelainen MK: U.S. Pat. No. 4,712,068, Sepponen RE: U.S. Pat. No. 4,587,493, Savelainen MK: U.S. Pat. No. 4,644,281 and Kupiainen J: U.S. Pat. No. 4,668,904.
Dynamic nuclear polarization (DNP) has been describe e.g. in the following references: Lepley AR and Closs GL: Chemically induced magnetic polarization, Wiley, New York 1973, Potenza J: Measurement and Applications of dynamic nuclear polarization, Adv. Mol. Relaxation Processes vol. 4, Elsevier, Amsterdam 1972, pp. 229-354, Ettinger KV: U.S. Pat. No. 4,719,425.
DNP is a magnetic double resonance method which thus requires two separate spin populations. Such spin populations include e.g. the spins of electrons and protons. In a double resonance method, the distribution of one spin population on various energy levels is varied and the other spin population is monitored. As certain conditions are fulfilled, the resonance signal of a spin population being monitored becomes amplified (Overhauserphenomenon). The amplitude of an amplified signal can be several hundred times higher than a non-amplified signal. The amplification factor may be positive or negative. The amplified signal is characteristically highly sensitive to the physico-chemical conditions and reactions of a spin environment, and, thus, has an obvious application for the examination of the chemical properties of a material.
The reference Ettinger KV: U.S. Pat. No. 4,719,425 discloses as applications the mapping of the contents of paramagnetic components and the mapping of the activity of cerebral nerve cells. In references Lurie DJ, Bussel DM, Bell LH, Mallard JR: Proton Electron Double Resonance Imaging: A new method for imaging free radicals, Proc. S.M.R.M. Fifth Annual Meeting, 1987, New York, p. 24 and Lurie DJ, Bussel DM, Bell LH, Mallard JR: Proton-Electron Double Magnetic Resonance Imaging of free radical solutions, J. Magn. Reson., vol. 76, 1988, pp. 366-370 discloses as possible applications the mappings of free radical groups, nitroxide radicals and oxidation degree.
A problem in the prior art is the absorption of ESR frequency electromagnetic oscillation in an object being examined. This leads to two major drawbacks: 1. The saturation on ESR frequency only occurs in those parts of an object which are near the radiator (for example, the penetration depth of 1.12 GHz in a muscular tissue is less than 3 cm). 2. Since the ESR line has a relatively great width, the saturation requires the use of high power which, on absorbing in an object, may result in the damage to the object (heating).
The interaction of electromagnetic radiation and biological tissue has been described e.g. in the following references: Roschmann P: Radiofrequency penetration and absorption in the human body: Limitations to high field whole body nuclear magnetic resonance imaging, Med. Phys. 14 (6), pp. 922-931, 1987, Tenforde TS and Budinger TF: Biological effects and physical safety aspects of NMR imaging and in vivo spectroscopy, in Thomas SR and Dixon RL (eds.) NMR in medicine: The instrumentation and clinical applications, Medical Physics Mcnograph No. 14, American Institute of Physics, New York 1986.
According to the reference Sepponen: U.S. Pat. No. 4,543, 959, it is prior known to combine NMR and ultrasonic imaging method. In the arrangement disclosed in the reference, the actual imaging is effected with ultrasound which is capable of real-time imaging and NMR examination is effected on a desired target area. The localization of a target or object area is effected by using methods known in magnetic imaging, such as a selective excitation in connection with magnetic field gradients as well as so-called Fourier methods for mapping the density distribution in the direction of a gradient field. A problem in the technical realization of this method is the generation of a relatively powerful magnetic field required by the NMR method in a manner that the ultrasonic examination can be readily effected.